Comprising two distinct and once connected structures, the Prison proper and the nearby Tower of the Wyrm, the former Faction headquarters of the Mercykillers, stands in Sigil in The Lady's Ward just spireward of the intersection of Prison Row, Rotten Row, and Guvner’s Mile at Couriers’ Square.
Fully three times the size of the City Barracks, the Prison stands 1,100 feet on a side between each of the four corner towers, with a further four towers of smaller stature arranged within each exterior wall. The walls stand seven stories high, and while the building looks solid enough from the exterior, the interior of the Prison is mostly a single giant courtyard. The interior of featureless gray and brown dirt is marked only by the two walkways that hang above the pit and allow for watching of prisoners on exercise.
Beyond the main gate of the Prison, which faces Courtiers’ Square, are the chambers formerly devoted to faction business, while the rest of the interior of the prison is composed mainly of cells for prisoners. While the prison has divested itself of many of the prisoners, it held formerly during the reign of Factol Alisohn Nilesia, the cells are slowly filling once more with the lawbreakers of Sigil, among them the more violent ones released on the misguided but good intentions of Arwyl+Swan’s+Son during the last days of the Faction War.
As dreary and oppressive as the main aboveground bulk of the Prison may appear to inmates or even a passerby, similar to the City Barracks, the worst was mostly unseen and unknown by the public ‘till recent years. It seems that below street level, carved out of the stone of Sigil’s ring, sat an underground level filled with solitary confinement cells for hardened prisoners as well as for random intimidation and psychological torture of simple inmates. The so-called Cellars, along with dining halls, laundry rooms, and other chambers for work detail for prisoners, were filled with the torture and sentencing chambers of the condemned and a poor sod had no way of knowing when on a daily basis he’d be descending to the Cellars for simple work detail, or to face the gallows.
The Prison is slowly being refit and rebuilt, and there are indications that the Sons of Mercy may attempt to officially petition to run the facility and to handle the incarceration of prisoners now currently being held in the Barracks and its adjacent buildings. This, of course, raises the hackles of more than a few council members, both for the giving of that official power to a faction and the proximity to the Sodkillers.
Adjacent to the Prison and standing within Petitioners’ Square rises the Tower of the Wyrm, a relic of Sigil’s past now serving as the headquarters of the Sodkillers and emblazoned on all sides still with the crimson dragon symbol of the Red Death. Inside, the tower was once filled with petty prisoners and interrogation rooms as well as the tower’s namesake, the Wyrm, or Cage Serpent. The Wyrm was a specially bred wyvern, magically enhanced by the Mercykillers to produce venom that induced delirium in prisoners when extracting confessions. Executions by hanging, as well as being simply devoured alive by the Wyrm, were commonplace during the reign of Nilesia, but since then the tower has not seen a public execution. It is unlikely that the Sigil Advisory Council will sanction any such actions by the Sodkillers.The Wyrm itself was slain during the peak days of the Tempest of Doors, though the body was never found and some say that it was either secreted back to Acheron by those still faithful to the faction’s ideals under Nilesia or to Nilesia herself, wherever she herself vanished to during the Faction War itself.
Sources: Text excerpted from Chapter 7 of Planewalker's 3e Planescape Campaign Setting material.
'Factols Manifesto', 'In the Cage: A Guide to Sigil'.